Sunstone v0.25.0

Added

• A Concept with sources in its Frontmatter shows them as a Sources list at
  the end of the document, numbered like the footnotes that cite them; sources
  not cited in the text follow without a number. The list is only shown, never
  written to the file.
• Hovering a footnote that cites a source, or a title in the Sources list,
  shows a card with the source's title, resource, author and dates right away,
  in the app and in the web viewer. Clicking it opens the source: another
  Concept in the app, or a web address in the browser.
• The Explorer expands the folders leading to the open Concept while it is
  open, and folds them back when you move elsewhere.